/Engine/Source/Runtime/MeshConversion/Public/MeshDescriptionBuilder.h
Append a UV triangle to the specified UV layer. This will use both shared and per-instance UV storage
void AppendUVTriangle ( const [FTriangleID](API\Runtime\MeshDescription\FTriangleID) & TriangleID, const [FUVID](API\Runtime\MeshDescription\FUVID) UVverterxID0, const [FUVID](API\Runtime\MeshDescription\FUVID) UVvertexID1, const [FUVID](API\Runtime\MeshDescription\FUVID) UVvertexID2, int32 UVLayerIndex )